1. Field of the Invention
The invention relates to a method of detecting and/or quantitatively analyzing hexacyanoferrates in an aqueous liquid.
As used herein, the term "hexacyanoferrates" encompasses both the hexacyanoferrate (II) ion (also referred to as ferrocyanide) and the hexacyanoferrate (III) ion (also referred to as ferricyanide).
2. Description Relative to the Prior Art
Hexacyanoferrates are widely used in many industries. They are used in the manufacture of blueprints, chemicals, detergents, textile dyes, mirrors, mortar, pesticides, pigments, and rubber. They are also used in case hardening, electroplating, photographic processing, mineral dressing, steel pickling, chemotherapy and the prevention of corrosion.
In such industries efforts are made to closely control the concentration of hexacyanoferrates in effluent streams because of environmental considerations. In effecting such control, it is desirable to be able to detect hexacyanoferrates, and in some instances to determine their concentration, in an aqueous liquid, and various methods are known for accomplishing this.
One group of such methods involves contacting a sample of the aqueous liquid with reagents which will react with hexacyanoferrates in acidic solutions to produce indicator compounds comprising blue precipitates such as Turnbull's Blue, Prussian Blue, and Berlin Blue. The reagents used are solutions containing ferric and/or ferrous ions. In acidic solutions these ions react with hexacyanoferrates to form complex salts (the blue precipitates). Such reactions are generally described, for example, in Cotton and Wilkinson, Advanced Inorganic Chemistry, N.Y., Interscience Publishers, 1966, pp. 860-1.
Descriptions of the use of such reactions to detect and quantitatively analyze hexacyanoferrates have also been published. For example, Chemical Abstracts, Vol. 88, 1978, p. 619, Abstract No. 88:98638d, Meditsch, "Semiquantitative determination of ferrocyanides," describes a method of detecting ferrocyanides by reaction with ferric salts impregnated in a filter paper test strip to form a precipitate of Prussian Blue (a complex ferric ferrocyanide salt). The color intensity of the precipitate is visually compared to similarly prepared standards in order to determine the concentration of ferrocyanide. Such a method suffers from lack of precision and sensitivity and from interference from extraneous precipitates which can form, depending upon the presence of other ions in the liquid being tested. For example, sulfur precipitates will also be formed if this method is used to test an aqueous effluent that contains thiosulfate ions, such as would commonly be encountered in photographic processing operations utilizing a ferricyanide bleach bath and a thiosulfate fixing bath. Such a method is also not readily applicable to an automated process for monitoring hexacyanoferrate concentration.
Chemical Abstracts, Vol. 78, 1978, p. 252, Abstract No. 140022u, Abramkina et al, "Determination of ferro- and ferricyanides in waste waters of the Leningrad Motion Picture Copying Plant," describes a method of quantitatively analyzing hexacyanoferrates in waste water by contacting a sample of the water with ferrous sulfate to form a precipitate of Turnbull's Blue (a complex ferrous ferricyanide salt) through reaction with ferricyanide and by contacting the sample with ferric chloride to form a precipitate of Berlin Blue (a complex ferric ferrocyanide salt) through reaction with ferrocyanide. The spectral densities of the precipitates of Turnbull's Blue and Berlin Blue are then measured in solution colorimetrically in order to determine the hexacyanoferrate concentrations. This method has the advantage over the filter paper method of being adaptable to automated on-line concentration monitoring through use of a flow cell colorimeter. The method suffers, however, from lack of sensitivity, accuracy, and precision and from interference by other precipitates (e.g., sulfur). Since the colored indicator compounds formed are themselves precipitates, they, along with any extraneous precipitates, tend to coat the flow-analysis apparatus, necessitating a complicated and expensive cleaning procedure. Furthermore, if the concentration of hexacyanoferrates in the water being analyzed is high enough (approximately 50 mg/L) the indicator precipitates will begin to clump together in the flow cell and cause the colorimeter measurements to become highly inaccurate (e.g., in many such cases the colorimeter readings will indicate much lower concentrations of hexacyanoferrates than are actually present in the sample).
Some of these problems can be avoided by other known methods of hexacyanoferrate analysis. Chemical Abstracts, Vol. 85, 1976, p. 474, Abstract No. 85:28154g, Yogo et al, "Spectrophotometric determination of ferro- and ferricyanide ions with 1,10-phenanthroline," describes a method of analyzing hexacyanoferrates in water at a pH of 2 to 4 by heating the water in the presence of a mercury catalyst and 1,10-phenanthroline and measuring the resultant spectral density of the solution. Chemical Abstracts, Vol. 73, 1970, p. 212, Abstract No. 69609e, Vail et al, "Photometric determination of ferrocyanides," describes a method of analyzing ferrocyanide in water at a pH of 4 by heating the water in contact with 2,2'-bipyridyl or 1,10-phenanthroline, formaldehyde, and an acetate buffer and measuring the resultant spectral density of the solution. The indicator compounds formed in these methods are the ferric and ferrous complexes of 1,10-phenanthroline and 2,2'-bipyridyl. Such complexes are soluble in aqueous liquids, and these methods avoid, therefore, the problems associated with the precipitating indicators formed in the other methods described above. Other problems remain, however, such as the likelihood that extraneous precipitates will be formed and interfere with the density measurements (e.g., if thiosulfate ions are present in a solution at a pH of 4 or less, sulfur precipitates can form). In addition, the heating with acid involved in the methods described in these two publications results in the decomposition of hexacyanoferrates to form ferrates and cyanides, whereafter the ferrate ions complex with the bipyridyl or phenanthroline, while the cyanide comes off as a gas which must be carefully handled and disposed of without significant escape into the environment.
Clearly, it would be desirable to have available a method of detecting and quantitatively analyzing hexacyanoferrates which: (1) exhibits high sensitivity, precision, and accuracy; (2) involves the formation of an indicator compound that is water-soluble; (3) avoids the formation of extraneous precipitates such as sulfur; (4) does not produce cyanide gas as a by-product; and (5) can be practically used in an automated, on-line, continuous-flow analytical system. The present invention provides such a method through the use of cobaltic complexes of tris-1,10-phenanthroline or tris-2,2'-bipyridyl which form water-soluble indicator complexes with hexacyanoferrates.
In regard to such complexes, it should be noted that American Cyanamid Company, Cyanamid's Nitrogen Chemicals Digest, N.Y., 1953, Vol. VII, The Chemistry of the Ferrocyanides, p. 48, briefly indicates that the cobaltous complex of ferrocyanide and tris-phenanthroline can exist as a colored precipitate in acidic liquids, but the publication describes no analytical method of using such a complex and does not describe cobaltic phenanthroline complexes or any uses thereof; i.e., its disclosure relates solely to a cobaltous complex rather than a cobaltic complex.